Ciclesonide is a novel inhaled corticosteroid delivered as inactive prodrug via a hydrofluoroalkane metered-dose inhaler with a deposition rate of 50 - 60 %. At its target sites, the lungs, ciclesonide is converted to an active metabolite, desisobutyryl-ciclesonide (des-CIC) [so-called on-site activation]. High lipophilicity and formation of local depot prolong pulmonary duration of action, explaining once-daily administration of ciclesonide. High protein binding and rapid clearance reduce systemic interactions. In long-term studies, ciclesonide at doses as high as 1280-1600 microg/d did not suppress biochemical markers of adrenal function. Since ciclesonide is not being activated in the oropharynx, the incidence of local adverse effects is comparable to that of placebo. Compared to other ICS, ciclesonide shows a improved therapeutic index and can, therefore, be regarded as prototype of a new, third generation of inhaled corticosteroids.